The wiring substrate used for transmitting high frequency signals is required to have excellent transmission characteristics, that is, small transmission delay and small transmission loss. In order to improve the transmission characteristics, it is necessary to use, as an insulating material of an electrical insulator layer of a wiring substrate, a material having a low dielectric constant and a low dielectric dissipation factor. As an insulating material having a low dielectric constant and a low dielectric dissipation factor, a fluororesin has been known.
As a wiring substrate using a fluororesin as an insulating material of an electrical insulator layer, for example, the following have been proposed.
(1) A high frequency wiring substrate having a dielectric layer of e.g. polytetrafluoroethylene (PTFE) formed on the surface of a metal substrate (Patent Document 1).
(2) A printed wiring board having an electrical insulator layer containing a fluorinated copolymer having acid anhydride residues and a conductor layer in contact with the electrical insulator layer (Patent Document 2).